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Reducing Compilation Overhead in J9/TR. Marius Pirvu, Derek Inglis, Vijay Sundaresan. Agenda. Adaptive compilation in the TR JIT Class Load Phase Asynchronous compilation Limiting the negative effect of very long compilations Ongoing work Improvements to async compilation AOT.

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Reducing Compilation Overhead in J9/TR

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Reducing Compilation Overhead in J9/TR

Marius Pirvu, Derek Inglis, Vijay Sundaresan


  • Adaptive compilation in the TR JIT

  • Class Load Phase

  • Asynchronous compilation

  • Limiting the negative effect of very long compilations

  • Ongoing work

    • Improvements to async compilation

    • AOT






Adaptive Compilation in TR JIT

  • Methods start out being interpreted

  • After N invocations methods get compiled at ‘warm’ level

  • Sampling thread used to identify hot methods

  • Methods may get recompiled at ‘hot’ or ‘scorching’ levels

  • Transition to ‘scorching’ goes through a temporary profiling step



How About Applications Without Hotspots

  • WebSphere AppServer startup

    • Very flat profile


  • Intuition: Methods compiled during startup phase may not be important during application run phase

  • Detect phases when class loading is intense

  • Reduce optimization level to “cold” during such phases


  • Synchronous compilation

    • Application thread places compilation request and blocks waiting for the compilation to finish

  • Asynchronous compilation

    • Application thread does not wait for the compilation result

  • JIT compilations performed on a separate compilation thread

Asynchronous Compilation

  • Implementation

    • Synchronous compilation still needed in some cases (e.g. pre-existence)

    • Synchronous and asynchronous compilation must coexist

    • Queue of compilation requests

  • Advantages

    • Takes advantage of available processors on SMP machines by increased parallelism

    • Allows performance improvements in uniprocessors by changing compilation thread priority

    • Allows reordering of compilation requests

Performance Results – Short Running Apps.

  • Asynchronous compilation on SMP reduces execution time of short benchmarks

Limiting Negative Effect of Long Compilations

  • Compilations may impede GC operation

    • GC requires exclusive VM access

    • Cannot allow class unloading while compilation in progress  Compilations require VM access

  • Solution

    • Compilation thread periodically releases and reacquires VM access allowing GC to cut-in

    • Upon re-acquiring VM access, check if GC unloaded any classes

    • If classes were unloaded, abort current compilation and retry

Ongoing Improvements to Asynchronous Compilation

  • Idea

    • Use thread priorities to smooth out the effects of compilation - effectively interleave compilation with execution

  • Implementation

    • Don’t use more than X% CPU for compilation

    • Use the queue of methods as a buffer

      • accumulate work during periods of heavy utilization

      • solve the backlog when CPU is lightly used or idle (due to IO for instance)

    • Prioritize compilation requests in the queue

Performance - Uniprocessors

Ahead Of Time (AOT) Compilation

  • Using the JIT as a static compiler

  • Fully compliant code

  • Used by J2ME customers to decrease footprint by eliminating the JIT

  • Will be used by the Real Time offering to eliminate the possibility of nondeterministic behaviour introduced by the JIT

  • Experimenting with combining AOT and JIT compilation to improve startup times

Eclipse 3.0.1 Startup Times

Hardware: 1P@1.2GHz Pentium3 M, Windows32 Options:-Xmx512m -Xms512m for all tests


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